Ceramic die nibs or inserts held within a steel casing for shaping of metal forced therethrough have been known for well over the past half-century. An early disclosure of such cased die nibs for wire drawing is in U.S. Pat. No. 1,096,688.
These ceramic die nibs have a propensity to suffer tensile stress failure in use. It was later found that this problem could be substantially alleviated by making the outside diameter of the nib very slightly larger than the inside diameter of the casing portion designed to surround the nib, then heating the casing to expand its noted inside diameter large enough to receive the nib (which is at room temperature), and lastly cooling the assembly to shrink-fit the casing onto and around the nib. Such shrink-fitted assembly placed the nib under sufficient compression to offset the tensile stress resulting from metal forced through a shaping aperture of the nib.
In order to avoid cracking of the nib during shrink-fitting into the casing, it became necessary to carefully grind and finish the outer circumferential surface of the nib to the same circumferential geometry as that of the surrounding inner circumferential surface of the casing, e.g. to concentricity between them. Otherwise, irregular or out-of-round mating between nib and casing caused uneven stresses in the nib that often produced early cracking of the nib (even during casing thereof).
However, the cost burden of such nib finishing work led to the discovery that such work could be substantially avoided by designing the assembly to accommodate a compressed intermediate layer of relatively low melting point glassy or resinous material between the shrink-fitted surfaces of the nib and casing as shown in U.S. Pat. No. 2,150,734. In addition to glassy or soft oxide material, U.S. Pat. No. 3,013,657 suggested the use of soft pure metal for the intermediate layer. U.S. Pat. No. 3,613,433 discloses relatively high-silica glaze or glassy materials which are incidentally shown between the shrink-fitted nib and casing. Because of their high SiO.sub.2 content, the glassy materials in this latter patent are believed to have somewhat higher softening points than suggested by the other noted patents, e.g. that of Glaze 7 is estimated to be about 1050.degree. C..+-.50.degree. C. and the lowest of the disclosed glazes or vitreous coatings. In each of these cases, the assembly design involved an intermediate layer of material that could be heated to a liquid type of flowable condition or melted at a temperature well below the melting temperatures of the nib and casing, and during shrink-fitting the heated flowable or melted material could easily fill the shrinking space between nib and casing. Moreover, such material was of a relatively plastic and gradually hardening (thermoplastic) nature as it cooled with the nib and casing. While cooling glassy and resinous materials undergo substantially continuous increase in viscosity to a rigid state at room temperature, cooling metals solidify to a relatively plastic state (subject to creep under pressure) that continues to get stiffer and more rigid as cooling continues. Thus, these earlier designs were also apparently predicated on some plastic flow of the intermediate material during cooling of the shrink-fitted assembly so as to further accommodate any irregular or out-of-round, outer, circumferential surface of the nib and its mating with the casing.
In the U.S. Air Force sponsored report ML-TDR-64-295 (or AD 608497) by Hunt et al. and dated Nov. 25, 1964, it is also proposed to insert a relatively softer metal (e.g. copper or aluminum) sleeve cushion between the shrink-fitted nib and casing without melting this cushion.
Ceramic die nibs of particular interest today are those made essentially of zirconia like those disclosed in U.S. Pat. No. 3,365,317 and the July 1, 1965 issue of The Iron Age magazine at pages 58-59.